Metal cans are fabricated by forming a flat metal blank, usually rectangular in shape, into a tubular configuration with the lateral ends or edges being lapped and welded together, defining a longitudinal seam. End closures are then secured across the open ends of the tubular configuration to complete the can formation. The term "tubular" is not restricted to a circular cross-section, as square or other shaped cans may be fabricated with this same approach. Also, similar continuously welded seams may be used for fabricating structural components other than cans.
One form of seam weld is made as opposed roller electrodes, one on the inside and one on the outside of the tubular configuration, continuously track along in the direction of the lapped ends. A large welding current is transmitted between the roller electrodes, via the small regions of contact defined between the opposing roller electrodes, between and through the lapped ends. The current is pulsed, to provide that the welded seam is actually comprised of a series of "spot welds", made closely adjacent one another.
With a tin-plated steel blank, a copper wire intermediate electrode, typically of rectangular cross-section, is fitted in a circumferential groove on each roller electrode, and pressed then against the opposite sides of the lapped ends. This serves to help carry the melted tin away from the welded seam region; although the tin does tend to solidfy and build up on the roller electrodes, and in the groove. Tin buildup increases the electrical resistance (compared to a new roller electrode), and reduces the effective welding current . . . resulting in erractic or even poor welds. This tin buildup periodically can be removed, by machining or "reprofiling" the periphery and groove of the roller electrode. As the roller electrodes must have specific minimum overall diameters and contact angles, reprofiling may be done only a limited number of times; thereafter, the roller electrode must be replaced. The steel blank could alternatively be zinc-coated, but the same buildup problems exist.
One form of inner roller electrode of this type commonly has a stator supported by the welding machine, and a rotor rotatably carried on the stator and having a circumferential guide groove for the copper wire. The rotor and stator are separated from one another by very small radial and circumferential clearance gaps (some possibly only 0.6 of a millimeter wide), across which the relative movement of the rotor and stator takes place. An electrically conductive liquid is sealed in the roller electrode, spanning substantial portions of the gaps, to conduct the welding current between the stator and rotor components. Appropriate bearings and surrounding insulators support relative rotation of the stator and rotor components, but otherwise electrically insulate these components from one another.
One form of roller electrode cooling provides for circulating a coolant liquid, commonly water charged with an anti-freeze, through passages defined in the stator. This cools the stator surfaces exposed to the electrically conductive liquid, and the electrically conductive liquid in turn then also serves to thermally cool the rotor.
The stator and rotor components of the roller electrodes are commonly formed of a copper alloy having a high content (possibly 98%) of copper, for yielding high electrical and thermal conductivity. Such an alloy also structurally resists deformation under the welding temperatures and pressures.
Modern welding equipment may weld with 6000 amperes of current at up to 40-50 kilowatts of power, giving a linear welding speed of 70 meters per minute, and yielding a production up to 600 cans per minute.
The electrically and thermally conductive liquid almost universally used in commerical roller electrodes has been mercury. Mercury remains a liquid to approxiamtely -38 degrees C, unequaled by any other conductive metal or eutectic mixture of metals, that is also stable at room temperatures. Mercury can carry the high welding currents needed in the roller electrode, and mercury can also provides sufficient cooling for the rotor.
Despite its wide use, mercury is not a willing first choice; in fact it has many very poor if not outright dangerous characteristics.
For example, mercury has electrical and thermal conductivities of approximately only 2% that of copper. The limited wetting ability of mercury adds to the reduced effectiveness of both electrical and thermal transmission between the stator and rotor components. Consequently, its presence: adds appreciably to the electrical current needed to generate the welding temperatures, which raises both the operating temperatures and cooling requirements; and gives off heat to the stator so poorly that the coolant may be heated only a few degrees in passing through the roller electrode, desipte being chilled to below room temperature, possibly between 5-15 degrees C. Moreover, the thermal expansion of mercury in the anticipated temperature range of use, 0-100 degrees C, is very large, so that complicated seals and/or overflow devices must be associated with the roller electrodes to accomodate this expansion.
Mercury is also very corrosive to the copper alloy stator and rotor components, producing an amalgam that limits both the shelf and operating lives of the roller electrode, to perhaps only several weeks. The amalgam, in its initial stage is gelatinous or paste-like, to increase drag against electrode rotation; whereas in its more advanced stages, it solidifies rock-hard to bind the componenets together completely. Once solidifed, it is typically impossible to dislodge the amalgam and disassemble the electrode components, such as for rebuilding and salvaging them for a second work cycle. The amalgam has poorer electrical and thermal conductivities than fresh mercury, correspondingly imposing ever higher welding currents and cooling demands.
Attempts to extend the shelf life provide that the mercury roller electrode may be maintained under refrigeration and/or repositioned frequently. Also, manufacturers may ship the roller electrodes empty, with a separate supply of the mercury the user must pour into the roller electrode and seal, when the need arises.
Toxicity of mercury however, remains probably its most significant drawback, from a liability standpoint. Mercury is frequently looked upon as a substance requiring special standards of care, and government approvals for its wide scale use and disposal. Such restrains detract from the appeal of having the user fill the roller electrode with the mercury and/or add appreciably to the overall costs associated with its use in roller electrodes. Mercury leaks to the environment, or even the threat of it, can be unsettling.
Other electrically conductive liquids have been proposed, to avoid the above-mentioned problems of mercury. However, such generally have not found commerical application, because the roller electrode had to be modified so much that it would not work in conventional welding machines; or the current carrying capacity of the liquid was inadequate for the high output welding demands.
A roller electrode is disclosed in U.S. Pat. No. 3,501,611; non-mercury electrically conductive liquids are disclosed in U.S. at. Nos. 4,188,523 (69.5 +or-5 Atomic % of gallium, 15.2 +or -1.0 Atomic % indium, 6.1 +or-1.0 Atomic % tin, 4.5+or-0.8 Atomic % zinc, 3.2 +or-0.5 Atomic % silver, and 1.5 +or-0.5 Atomic % aluminum; 4,433,229 (pure gallium, or bianary metals or gallium including gallium/indium and gallium/tin); and U.S. Pat. No. 4,642,437 discloses a colbalt coating on the roller electrode. Foreign patents of interest might include West Germany Pat. Nos. 2,351,534 (Beck); 2,805,345 (Janitzka) and 3,432,499 (Lorenz); Swiss No. 597,971; and Japanese No. 0001583, disclosing different conductive liquids of gallium and/or different overlying coatings on the roller electrode.